19%‐efficient and 43 µm‐thick crystalline Si solar cell from layer transfer using porous silicon

We present a both‐sides‐contacted thin‐film crystalline silicon (c‐Si) solar cell with a confirmed AM1.5 efficiency of 19.1% using the porous silicon layer transfer process. The aperture area of the cell is 3.98 cm². This is the highest efficiency ever reported for transferred Si cells. The efficiency improvement over the prior state of the art (16.9%) is achieved by implementing recent developments for Si wafer cells such as surface passivation with aluminum oxide and laser ablation for contacting. The cell has a short‐circuit current density of 37.8 mA cm⁻², an open‐circuit voltage of 650 mV, and a fill factor of 77.6%. Copyright © 2011 John Wiley & Sons, Ltd.     

Analysis of short circuit current gains by an anti‐reflective textured cover on silicon thin film solar cells

The influence of a retro‐reflective texture cover on light in‐coupling and light‐trapping in thin film silicon solar cells is investigated. The texture cover is applied to the front glass of the cell and leads to a reflectance as low as r ≈ 3% by reducing the reflection at the air/glass interface and indirectly also reducing the reflections from the internal interfaces. For weakly absorbed light in the long wavelength range, the texture also enhances the light‐trapping in the solar cell. We demonstrate an increase of the short circuit current density of exemplary investigated thin film silicon tandem solar cells by up to 0.95 mA cm⁻² and of the conversion efficiency by up to 0.74% (absolute). For a planar microcrystalline solar cell, the enhancement of light‐trapping was determined from the reduced reflection in the long wavelength range to be up to 17%, leading to an increase of the external quantum efficiency of up to 12%. Copyright © 2012 John Wiley & Sons, Ltd.     

标准测试条件下人造太阳光源的谱失配及影响

标准测试条件下,人造太阳光源的光谱形式、辐 照强度、光谱失配因子等是影响太 阳电池器件测试结果准确性的重要因素。本文以标准太阳光谱AM1.5 为参考,从光谱失配角 度,计算和分析了4种常用人造太阳光源(Arc lamp灯、Q-Flash灯、Q-Flash W灯和ELH 灯)与标准太阳光谱AM1.5之间的光谱失配因子的变化和这四种人造 太阳光源辐照下晶硅电 池的输出参数的变化。计算结果表明:Arc lamp型人造太阳光源的光谱失配因子为 0.979, 晶硅电池输出参数最接近标准太阳光谱AM1.5辐照下的输出参数 ;因光谱失配影响,4种不同人造太阳光源辐照下晶硅电池的输出参数较标准太阳光谱AM1.5辐照下的输出 参数会发生明显变化。     

New Family of Ruthenium‐Dye‐ Sensitized Nanocrystalline TiO2 Solar Cells with a High Solar‐Energy‐Conversion Efficiency

A new type of ruthenium complexes 6 – 8 with tridentate bipyridine–pyrazolate ancillary ligands has been synthesized in an attempt to elongate the π‐conjugated system as well as to increase the optical extinction coefficient, possible dye uptake on TiO₂, and photostability. Structural characterization, photophysical studies, and corresponding theoretical approaches have been made to ensure their fundamental basis. As for dye‐sensitized solar cell applications, it was found that 6 – 8 possess a larger dye uptake of 2.4 × 10^–7 mol cm^–2, 1.5 × 10^–7 mol cm^–2, and 1.3 × 10^–7 mol cm^–2, respectively, on TiO₂ than that of the commercial N3 dye (1.1 × 10^–7 mol cm^–2). Compound 8 works as a highly efficient photosensitizer for the dye‐sensitized nanocrystalline TiO₂ solar cell, producing a 5.65 % solar‐light‐to‐electricity conversion efficiency (compare with 6.01 % for N3 in this study), a short‐circuit current density of 15.6 mA cm^–2, an open‐circuit photovoltage of 0.64 V, and a fill factor of 0.57 under standard AM 1.5 irradiation (100 mW cm^–2). These, in combination with its superior thermal and light‐soaking stability, lead to the conclusion that the concomitant tridentate binding properties offered by the bipyridine‐pyrazolate ligand render a more stable complexation, such that extended life spans of DSSCs may be expected.     

17.6% efficient tricrystalline silicon solar cells with spatially uniform texture

Up to now solar cells fabricated on tricrystalline Czochralski‐grown silicon (tri‐Si) have shown relatively low short‐circuit current densities of about 31–33 mA/cm² because the three {110}‐oriented grains cannot effectively be textured by commonly used anisotropic etching solutions. In this work, we have optimised a novel chemical texturing step for tri‐Si and integrated it successfully into our solar cell process. Metal/insulator/semiconductor‐contacted phosphorus‐diffused n⁺p junction silicon solar cells with a silicon‐dioxide‐passivated rear surface and evaporated aluminium contacts were manufactured, featuring a spatially uniform surface texture over all three grains on both cell sides. Despite the simple processing sequence and cell structure, an independently confirmed record efficiency of 17.6% has been achieved. This excellent efficiency is mainly due to an increased short‐circuit current density of 37 mA/cm² obtained by substantially reduced reflection and enhanced light trapping. Copyright © 2003 John Wiley & Sons, Ltd.     

Sn‐catalyzed silicon nanowire solar cells with 4.9% efficiency grown on glass

We present a single pump‐down process to texture hydrogenated amorphous silicon solar cells. Mats of p‐type crystalline silicon nanowires were grown to lengths of 1 µm on glass covered with flat ZnO using a plasma‐assisted Sn‐catalyzed vapor‐liquid‐solid process. The nanowires were covered with conformal layers of intrinsic and n‐type hydrogenated amorphous silicon and a sputtered layer of indium tin oxide. Each cell connects in excess of 10⁷ radial junctions over areas of 0.126 cm². Devices reach open‐circuit voltages of 0.8 V and short‐circuit current densities of 12.4 mA cm⁻², matching those of hydrogenated amorphous silicon cells deposited on textured substrates. Copyright © 2012 John Wiley & Sons, Ltd.     

Hybrid Solar Cells Using HgTe Nanocrystals and Nanoporous TiO2 Electrodes

HgTe nanocrystals are demonstrated to increase the photon‐harvesting efficiency of hybrid solar cells over a broad spectral region between 350 and 1500 nm. Devices combining two solar cell concepts, a solid‐state nanocrystal‐sensitized solar cell and a nanocrystal/polymer‐blend solar cell, are described. These devices give incident photon to current efficiencies up to 10 % at around 550 nm monochromatic irradiation and short‐circuit current densities of 2 mA cm^–2 under simulated AM1.5 (100 mW cm^–2) illumination (AM: air mass).     

Heterojunction solar cells with 23% efficiency on n‐type epitaxial kerfless silicon wafers

We present a heterojunction (HJ) solar cell on n‐type epitaxially grown kerfless crystalline‐silicon with an in‐house‐measured conversion efficiency of 23%. The total cell area is 243.4 cm². The cell has a short‐circuit current density of 39.6 mA cm⁻², an open‐circuit voltage of 725 mV, and a fill factor of 0.799. The effect of stacking faults (SFs) is examined by current density (J) mapping measurements as well as by spectral response mapping. The J mapping images show that the localized lower J regions of the HJ solar cells are associated with recombination sites originating from SFs, independent of whether SFs are formed on the emitter or absorber side. The solar cell results and our analysis suggest that epitaxially grown wafers based on kerfless technology could be an alternative for low‐cost industrial production of Si HJ solar cells. Copyright © 2016 John Wiley & Sons, Ltd.     

Fabrication of monolithic photovoltaic arrays on crystalline silicon by wafer bonding and deep etching techniques

In this work, a novel technology to fabricate small (∼1 cm²) c‐Si photovoltaic mini‐modules is shown. This technology combines two main bulk micro‐machining techniques: fusion (or adhesive) bonding and anisotropic etching of silicon. Due to the fact that the photovoltaic cells are fabricated in the same wafer, it is mandatory to etch the whole substrate to ensure electrical isolation. Once the individual cells are bulk‐isolated they can be connected in series so as to scale up the output voltage of the mini‐array. A handling wafer is required to provide mechanical stability to the device wafer. Adhesive and fusion bonding are used to join the handling and the device wafer. First electrical results, under standard Air Mass 1·5 (AM 1·5) solar spectrum light (100 mW/cm²), using a 9‐cell series connected mini‐module fabricated by fusion bonding, leads to a total open‐circuit voltage of 4·11 V, a short‐circuit current of 2·45 mA, and a maximum delivered power of 3·8 mW for each mini‐module (1·4 cm²). A 16‐cell series‐connected mini‐module fabricated by adhesive bonding and wire bonding, yields an open‐circuit voltage of 7·45 V, a short‐circuit current of 390 µA, and maximum delivered power of 1·8 mW, with 1·1 cm² of mini‐module area. Copyright © 2005 John Wiley & Sons, Ltd.     

Light trapping in pyramidally textured crystalline silicon solar cells using back‐side diffractive gratings

Back‐side diffractive gratings enhance a solar cell's efficiency by trapping light inside the cell and increasing the probability of absorption. We introduce a three‐dimensional, polarization‐sensitive optical model combining ray tracing and rigorous coupled‐wave analysis to investigate silicon solar cells with pyramidal front‐side texturing and back‐side gratings. Parameter optimization is performed to increase the short‐circuit current density for a linear binary grating with grating period p and height h. For the investigated 180‐µm‐thick pyramidally textured silicon solar cells, the simulation yields a maximum enhancement of the short‐circuit current density by ΔJ_SC = 1.79 mA/cm² corresponding to an absolute efficiency increase of Δη = 0.90%. Furthermore, we report on fabrication and reflectance measurements of solar cells with gratings and key challenges in achieving efficiency gains using back‐side diffractive gratings. Copyright © 2012 John Wiley & Sons, Ltd.     

Transparent conductive oxide‐less three‐dimensional cylindrical dye‐sensitized solar cell fabricated with flexible metal mesh electrode

A cylindrical transparent conductive oxide‐less dye‐sensitized solar cell (DSSC) consisting of glass tube/stainless steel mesh–TiO₂–dye/gel electrolytes/Pt‐Ti rod having capability of self‐light trapping is reported. Replacing the glass tube with heat‐shrinkable tube to reduce electrolyte gap and optical loss due to light transmission and reflection led to the enhancement in the power conversion efficiency from 2.61% to 3.91%. Profiling of the current distribution measured by laser beam‐induced current exhibited nearly the same current in the axial and radial directions, suggesting that light reflection on a cylindrical DSSC does not affect the efficiency seriously. Optimized best DSSC in this novel device architecture gave a short‐circuit current density of 11.94 mA/cm², an open‐circuit voltage of 0.71 V and a fill factor of 0.66 leading to the power conversion efficiency of 5.58% at AM 1.5 under simulated solar irradiation. Copyright © 2011 John Wiley & Sons, Ltd.     

Increase in external quantum efficiency of encapsulated silicon solar cells from a luminescent down‐shifting layer

This paper reports the external quantum efficiency (EQE) of encapsulated screen‐printed crystalline silicon solar cells, where the encapsulation includes a layer of luminescent down‐shifting (LDS) molecules. At wavelengths less than 400 nm, the inclusion of the LDS molecules increases the EQE from near zero to, at most, 40%. The increase in EQE corresponds to a rise in short‐circuit current density of 0·37 ± 0·13 mA/cm² under the AM1‐5g spectrum. Copyright © 2008 John Wiley & Sons, Ltd.     

Solar cells with 11% efficiency on ribbon-growth-on-substrate (RGS) silicon

Solar cells have been prepared on Bayer ribbon-growth-on-substrate (RGS) crystalline silicon. This low-cost material contains a fair amount of impurities and crystal defects whose adverse effect on solar cell performance may be significantly reduced by gettering and bulk passivation treatments. This is demonstrated in solar cells by having a mechanical surface texturization, an aluminium gettering as well as a hydrogen passivation step which led to an open circuit voltage V_oc of 538 mV, a short circuit current density J_sc of 28·5 mA cm⁻², a fill factor of 72·4% and a confirmed record efficiency β of 11·1% (2×2 cm²). Strong improvements in the diffusion length could be observed after the hydrogen treatment. © 1998 John Wiley & Sons, Ltd.     

Photocurrent enhancement in thin‐film silicon solar cells by combination of anti‐reflective sub‐wavelength structures and light‐trapping textures

Dielectric films with anti‐reflective sub‐wavelength structures are applied to thin‐film silicon solar cells to improve the light incoupling at the front surface. It is verified that modification of the refractive index of the incident medium using dielectric films with sub‐wavelength structures is beneficial to reduce the average reflectivity of Si solar cells with an anti‐reflective coating based on optical interference. It is also shown that the sub‐wavelength structure must be combined with a proper light‐trapping texture to enhance the absorption within thin‐film silicon solar cells. The effectiveness of dielectric films with sub‐wavelength structures is demonstrated by an increase of the short‐circuit current density of a microcrystalline silicon cell from 29.1 to 30.4 mA/cm² in a designated area of 1 cm². The optical interplay between the dielectric films and the light‐trapping textures is also discussed. Copyright © 2015 John Wiley & Sons, Ltd.     

High‐irradiance degradation tests on concentrator GaAs solar cells

The present work summarises the results of an experiment of light‐soaking high‐concentrator MOVPE‐grown GaAs solar cells under monochromatic light (808 nm). The irradiance level was set so that the short‐circuit current obtained was 1100 times that produced with the AM1ċ5D spectrum at 1 kW/m². This test caused no morphological changes in the devices. The main phenomenon discovered has been a slight increase with time of the reverse current I₀₂. This increase is analogous to that observed in similar degradation experiments based on high forward currents. In general, the results of these tests show that the drop in performance is very limited, supporting the idea that concentrator GaAs solar cells are rugged devices, capable of achieving long lifetimes in field operation. Copyright © 2003 John Wiley & Sons, Ltd.     

A self‐calibrating led‐based solar test platform

A compact platform for testing solar cells is presented. The light source comprises a multi‐wavelength high‐power LED (light emitting diode) array allowing the homogenous illumination of small laboratory solar cell devices (substrate size 50 × 25 mm) within the 390–940 nm wavelength range. The spectrum can be synthesized by independent tuning of the 18 different wavelengths to mimic AM1.5G as well as various indoor lamp spectra. The intensity can be controlled with a 2¹⁴‐bit accuracy and intensities up to 3 suns are possible with an approximate AM1.5G spectral distribution. For several wavelengths intensities up to 10 suns is possible, and for a few wavelengths up to 30 suns can be reached. The setup is equipped with reference diodes and an optical fibre coupling enabling calibration, monitoring and control of the light impinging on the sample. Through a computer controlled interface, it is possible to perform all the commonly employed measurements on the solar cell at very high speed without moving the sample. In particular, the LED‐based illumination system provides an alternative to light‐biased incident photon‐to‐current efficiency measurement to be performed which we demonstrate. Both top and bottom contact is possible and the atmosphere can be controlled around the sample during measurements. The setup was developed for the field of polymer and organic solar cells with particular emphasis on enabling different laboratories to perform measurements in the same manner and obtain a common basis for comparing data. The use of the platform is demonstrated using a standard P3HT:PCBM polymer solar cell but is generally applicable to any solar cell technology with a spectral response in the 390–950 nm region. Copyright © 2010 John Wiley & Sons, Ltd.     

Autodiffusion: a novel method for emitter formation in crystalline silicon thin‐film solar cells

The in situ formation of an emitter in monocrystalline silicon thin‐film solar cells by solid‐state diffusion of dopants from the growth substrate during epitaxy is demonstrated. This approach, that we denote autodiffusion, combines the epitaxy and the diffusion into one single process. Layer‐transfer with porous silicon (PSI process) is used to fabricate n‐type silicon thin‐film solar cells. The cells feature a boron emitter on the cell rear side that is formed by autodiffusion. The sheet resistance of this autodiffused emitter is 330 Ω/□. An independently confirmed conversion efficiency of (14·5 ± 0·4)% with a high short circuit current density of (33·3 ± 0·8) mA/cm² is achieved for a 2 × 2 cm² large cell with a thickness of (24 ± 1) µm. Transferred n‐type silicon thin films made from the same run as the cells show effective carrier lifetimes exceeding 13 µs. From these samples a bulk diffusion length L > 111 µm is deduced. Amorphous silicon is used to passivate the rear surface of these samples after the layer‐transfer resulting in a surface recombination velocity lower than 38 cm/s. Copyright © 2006 John Wiley & Sons, Ltd.     

Two‐dimensional simulation of thin‐film silicon solar cells with innovative device structures

We investigate the optical and electrical properties of thin‐film silicon solar cells by means of numerical simulations. The optical design under investigation is the encapsulated‐V texture, which is capable of absorbing sunlight corresponding to a maximum short‐circuit current density of 35 mA cm⁻². Because the layer thickness can be restricted to only 4 μm, the encapsulated‐V structure also provides a good collection efficiency for photogenerated charge carriers. The results for our simulations suggest that practical efficiencies above 12% can be expected for Si material with a minority carrier lifetime as low as 10 ns. Increased lifetimes of 100 ns allow for about 14% efficiency. The benefit of multiplejunctions within the device structure strongly depends on surface recombination. The efficiency of a single‐junction cell can be improved by more the 3% absolute with a multi‐junction device if the surface combination velocity is as high as 10⁵ cm s⁻¹. For moderate surface recombination, the gain is only 1%. Copyright © 1999 John Wiley & Sons, Ltd.     

Impact of phosphorus gettering parameters and initial iron level on silicon solar cell properties

We have studied experimentally the effect of different initial iron contamination levels on the electrical device properties of p‐type Czochralski‐silicon solar cells. By systematically varying phosphorus diffusion gettering (PDG) parameters, we demonstrate a strong correlation between the open‐circuit voltage (V_oc) and the gettering efficiency. Similar correlation is also obtained for the short‐circuit current (J_sc), but phosphorus dependency somewhat complicates the interpretation: the higher the phosphorus content not only the better the gettering efficiency but also the stronger the emitter recombination. With initial bulk iron concentration as high as 2 × 10¹⁴ cm⁻³, conversion efficiencies comparable with non‐contaminated cells were obtained, which demonstrates the enormous potential of PDG. The results also clearly reveal the importance of well‐designed PDG: to achieve best results, the gettering parameters used for high purity silicon should be chosen differently as compared with for a material with high impurity content. Finally we discuss the possibility of achieving efficient gettering without deteriorating the emitter performance by combining a selective emitter with a PDG treatment. Copyright © 2012 John Wiley & Sons, Ltd.     

On the interplay of cell thickness and optimum period of silicon thin‐film solar cells: light trapping and plasmonic losses

Light trapping and photon management in honeycomb‐textured microcrystalline silicon solar cells are investigated experimentally and by modeling of the manufacturing process and the optical wave propagation. The solar cells on honeycomb‐textured substrates exhibit short circuit current densities exceeding 30 mA/cm² and energy conversion efficiencies of up to 11.0%. By controlling the fabrication process, the period and height of the honeycomb‐textured substrates are varied. The influence of the honeycomb substrate morphology on the interfaces of the individual solar cell layers and the quantum efficiency is determined. The optical wave propagation is calculated using 3D finite difference time domain simulations. A very good agreement between the optical simulation and experimental results is obtained. Strategies are discussed on how to increase the short circuit current density beyond 30 mA/cm². In particular, the influence of plasmonic losses of the textured silver (Ag) reflector on the short circuit current and quantum efficiency of the solar cell is discussed. Finally, solar cell structures with reduced plasmonic losses are proposed. Copyright © 2015 John Wiley & Sons, Ltd.